Control system utilizing a minimum excitation control device



July 1, 1952 D. J. SIKORRA CONTROL SYSTEM UTILIZING A MINIMUM EXCITATION CONTROL DEVICE Mme/21m Filed Dec.

?atented July 1 1 952 CONTROL SYSTEM UTILIZING A MINIMUM EXCITATION CONTROL DEVICE Daniel J. Sikorra, Milwaukee, -Wis., assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis., a corporation of Delaware Application December 30, 1949, Serial No. 136,026

Claims. 1

This invention relates in general to regulating systems, and in particular to regulating systems utilizing a minimum excitation control device.

A minimum excitation control device operates to supply a voltage to the excitation controlling means of a dynamoelectric machine in a direction tending to increase the excitation of the machine when the excitation of the regulated machine tends to decrease below a predetermined minimum value. When the device is used in connection with a voltage regulator \which supplies to the excitation controlling means a control voltage having a polarity and magnitude dependent upon the direction and magnitude of variations in the output voltage of the machine from a predetermined value, it is desirable to limit the amplitude of the control voltage when the control voltage has the polarity which causes the excitation of the machine to decrease, in order to limit the voltage which the voltage supplied by the minimum excitation control device must overcome.

Prior regulating systems sometimes utilized the relation between the watts output of the machine and the excitation of the machine to provide minimum excitation control since this relation is a measure of the stability of a dynamoelectric machine. Such systems balanced a voltage which was a measure of the watts output of the regulated machine against a voltage which was a measure of the excitation of the machine, and utilized the diiierential voltage, only when the former voltage exceeded the latter, to increase the excitation of the machine.

Such systems have the disadvantage that the relation between watt output and minimum excitation required for stability varies according to the voltage of the line being supplied by the generator. That is, for a given watt output of the machine, the minimum excitation required for stability of the machine increases when the line voltage decreases, and conversely. This necessitates that the two balanced voltages mentioned above be adjusted each time the system voltage is changed by the action of tap changers or other causes, in order to maintain accurate minimum excitation control or the machine at all times.

These disadvantages can be overcome by providing a minimum excitation control device which is independent of the output voltage of the machine and hence not aiiected by variations in this voltage. The relation between the inphase component of current supplied by the machine and the excitation of the machine, for

2 stable operation of the machine, is a linear one, and by utilizing a voltage which is a measure of this in-phase component of current in place of the voltage proportional to the watts output of the machine, a minimum excitation control device which is independent of the output voltage of the machine may be obtained.

t is therefore an object of this invention to provide a dynamoelectric machine regulating system utilizing a minimum excitation control device associated with a regulator which limits the maximum amplitude of a control voltage tending to decrease the excitation of the machine.

It isa further object of this invention to provide a dynamoelectric machine regulating system utilizing a minimum excitation control device which is responsive to the, in-phase component of the current supplied by the machine.

It is an additional object of this invention to provide a dynamoelectric machine regulating system utilizing a minimum excitation control device which is independent of the output voltage of the machine.

Objects and advantages other than those stated above will be apparent from the following description when read in connection with the accompanying drawing, in which:

Fig. 1 is a diagrammatic representation of a regulating system embodying the apparatus and circuit of this invention;

Fig. 2 is a graph, the curves of which represent the relationship between the voltages of the voltage responsive regulator of the invention;

Fig. 3 is a diagram vectorially showing the relationship between the electric quantities employed in the minimum excitation control device of the invention; and

Fig. 4 is a graph showing'the relationship between the minimum excitation of the regulated machine and a characteristic of the current supplied by the machine for stable operation.

Referring to Fig. 1, the invention is shown as applied to the regulation of a dynamoelectric machine 5 which supplies an alternating current load circuit represented by conductors 6, l and 8. Machine 5 is provided with a field winding 9, the energization of which is supplied by a main exciter generator l0. Main exciter H) is provided with a field winding II which is energized by an exciter generator l2. The energizetion of a field winding 13 of exciter I2 is supplied by an excitergenerator M. Field winding I5 of exciter I4 is connected to be responsive to the voltage of machine 5 and to the excitation of field winding 9, to generally maintain the volttude of variations in the voltage of machine from a predetermined value. The regulator may be of any suitable type, but is preferably of the,

kind disclosed and claimed in my copending application, Serial No. 114,751, filed September 9,

1949, now Patent No. 2,576,646. Thisregulator comprises a plurality of nonlinear resonant circuits, each circuit comprising the primarywinde ing 25 of a saturable transformer 26 connected in series with a capacitor 27. Each winding 25 may be shunted by small capacitor 32 for the purpose of sharpening the point of resonance-of;

the'nonlinear circuits. One such nonlinear circuit is provided in each phase of the system and the circuits are energized bya measure of, the voltage of machine .l'through transformers l'l connected to conductors 6, l 'and' 8, autotransformers l9 and booster transformers 2|.

Each saturable transformer 281s provided with a secondary winding Zi'for furnishing a voltage which is proportional to the voltage of its asso ciated primary winding 25. connected to a resistor 34 through voltage reduc-: ing resistors 31 and a three phase rectifier 33 to impress on resistor 35 a rectifiedvoltage proportional to the average of the voltages of the capam itors 21- A three phase rectifier 35 isconnected' to secondary windings 29 of transformers 26 to produce across a resistor 31a rectified voltage which is proportional to the average-of the voltages of windings 29.

I5 of exciter Id is connected across-resistor 34 and an adjustable portion of resistor 31 through an'adjustable tap on resistor 37;.andis thereby directionallyenergized by a control voltage-equal to the difference between the'voltages of resistor 34-and of a portion of resistor 31.

A resistor 36in the circuit of field windinglS;

serves to lower the time constant'of this circuit and thereby accelerate the-responseiofexciter M to variations in the differential voltage. Any

suitable damping means, such as atransformer S5 havingaprimary winding fifi'connected across exciter l9 and a secondary winding 21 connected in the circuit of field winding lz'a may' be utilized to prevent hunting of the-system. V

To provide means for'limitingth'e amplitude of a control voltage of a polarity to decrease the? excitation of field winding li a rectifier A tend a filtering reactor 39-are connected in series across the outer terminals of rectifiers 33 and'35f' Rec tifier id is so poled as to block the fiow'of current when the voltage of resistor 31 exceeds the-volt age of resistor 36. When the voltage or -resistor- 34 exceeds the voltage of resistor 3l,rectifie'r 44 substantially short circuits resistors 34 -and 3l, thereby substantially equalizingthe voltages-of the resistors. maximum amplitude of the control voltage to reduce the excitation of machine 5:

It will be apparent that if field winding 1 5 'vvere connected across the noncommonterminals of resistors 34 and 31, rectifier 44 would-institute its- Capacitors 2? are Becauseof th'erconnections of rectifiers 33 and 3%,- the voltage of resistor 34' opposes'the voltage of resistor 31; Fieldwvinding:

This has the efiect of limitingthe 70 applied to field winding l5 in a direction tending limiting action when zero voltage exists on field winding I5. However, asa result of the connection of field winding l5, as shown, to a terminal of resistor 34 and to the adjustable tap on resistor 31, when the limiting action of rectifier 44 takes place, the voltage impressed on field winding l5 has reversed and reached. EL -predetermined value to permit limited reverse forcing of field winding I5.

The variations in the voltages across capacitors 21 and secondary windings 26 in function of variations-of-the output voltage of machine 5 are shown by the curves of Fig. 2. Curve 4| of Fig. 2 represents the portion of the rectified voltages of "capacitors 2 l which is impressed on resistor 34. Curve d2 represents the rectified voltage of transformer secondary windings 25 as it appears across the portion of resistor 31 adjacent resistor 34; Because of the nonlinearity of the circuit comprising a winding 25- and a capacitor 21, at lowvalueslof linevoltage themajor partofath'e voltageproportional to'the voltage .of machinei',-

which is. impressed on windings 25.andcapa'citor I 2'1, isabsorbed by windings 25. Hence, aseth'e voltage of machine 7 5' is increased from zero, the voltage-of windings 25zincreases steadily. as shown-.- by; curve 42in Fig; 2, while'the voltage of capacitorszz'i remains at a low value as shown by thei lowerportion of curvetl, and difierssufiicientlyi little from the value zero as I to be: without-an appreciable eifect onztheoperation of 'the system-r However, at a predetermined value'iofthe;volt-- age of 'machine' 5 the nonlinear circuits become: resonant; At thispredetermined' value :the.-.voltage ofcapacitors 27 suddenly increases'ras'shownl by the steep portion ofcurve 4 I, while theyoltagee of windings 25 1 remains substantially constant: becanse of the"saturationof.transformersLZBr A" resistor 28may be connected across eachi'capacitor:2l to'adjust .theslope of this portion'ofcurve'v 4 I. Since thevoltageiof each secondaryzwindinglz 29 is directly proportional to the voltagezof associated primary winding 25," curve 5 42 "mayi be considered as proportionally f representing? the voltageof secondary windings 29Iaswell astprie mary windings .2 5..

Curve 43, obtained byi'subtractin'g ithe iordinatesr: of curve-M from those of curve 42,-represents' the 1 differential voltage applied'to field winding'tl 5 by; the regulator at varying values of the=:voltage.of machine 5.

In: accordance with this invention, meanslare'i provided for maintainingthe excitati'on'offield winding 9 above a variable:predetermihedEmini mum value. Such means'include a three phase transformer 49 having. its primary" phase wind; ings '52 connected to load conductors'fi; E an'dfifl to be thereby energizedfby' a Voltage which is a measure of the output voltage of machine: 5: Transformer 69 is provided with secondary'phasewindings 53a, 53b and 530, of which windings" 53b, 530 only are connected in star. The-ratio between the turns of the primary and secondary windings of transformer 49 may be of any suit able value, in accordance with the value ofithe voltageof conductors 8, land 8.

An autotransformer 54 has its terminals' con nected to the noncommon terminals of the sec-i ondary'phase windings 53b, 530 to produce across autotransformer 54 a voltage which is the resultant of the voltages of the two interconnected secondary phase windings 53band 53c andwhich: is'in-phasevvith the phasevoltage of conductors-7i A- current transformer 55,-. placed in series with-.1 the load conductor '1, produces.across;anyrsuiteable, impedance means such as a resistor 56, a voltage which is a measure of the current in load conductor I. Owing to the connection of the secondary phase windings 53b, 530, the voltage of resistor 56 is in phase with the voltage of autotransformer 54 when the output of machine 5 is at unity power factor.

Resistor 56 has one of its terminals connected to a center tap on autotransformer 5i and the other of its terminals connected to the common terminal of the primary windings 51, 53 of two transformers, providedwith secondary windings 59 and Bi. Windings 57 to El all have the same number of turns. The terminals of autotransformer 5d are connected to the noncommon terminals of primary windings 5'1, 58. The voltage of secondary winding 59 will therefore be equal to the vectorial sum of the voltage of resistor 56 and one-half of the voltage of autotransformer 54, and the voltage of secondary winding 6| will be equal to the vectorial difference between-onehall of the voltage of autotransformer 5d and the voltage 01 resistor 55. Secondary winding 59 is connected to the alternating current terminals of any suitable known full wave rectifier 62, and winding El is connected to the alternating current terminals of a similar fullwave rectifier B3. A resistor ii is connected to the direct current terminals of rectifier 83 and a resistor it! is connected. to the alternating'current terminals of rectifier 32.

Fig. 3 shows the relation between the voltage vectors of the circuits of the minimum excitation control device described thus far. Vector AB represents the voltage of secondary phase winding 53b and vector BC represents the voltage of secondary phase winding 53c. Vector AC is the resultant of vectors AB and BC and represents the voltage of autotransformer 54, since autotransformer 54 is connected across the noncommon terminals of windings 53b and 530. Point D on voltage vector AC represents the center tap of autotransi'ormer st to which one terminal of resistor 58 is connected.

The voltage of resistor 55%; which is a measure of the current in conductor '5 of the load circuit, is represented by vector BE in phase with the voltage vector AC when the output of machine 5 is at unity power factor. The voltage of resistor 56 is represented by another vector DF when the current in the load circuit leads the output voltage of machine 5 by an angle 6. Owing to the connection of autotransformer 5G, resistor 55 and windings 5'1, 58, at unity power factor in the output circuit of machine 5, the voltage of transformer secondary winding 59 is represented by the vector AE and the voltage of secondary wind ing til is represented by the vector-EC. Since rectifiers (i2 and 63 are connected to secondary windings 59 and iii, respectively, it will be apparent that the diilerence between the voltages of rectifiers 62 and 63 at unity power factor will be a voltage equal to twice sistor 5%.

At a power factor angle of 6 in the output circuit of machine 5, the voltage 01" rectifier 62 may be represented by vector AF and the voltage of rectifier 63 by vector FC. The component of voltage DF which is in phase with the voltage AC may be obtained by projecting vector DF at DG on vector AC. By making voltage AC large compared with voltage DF, the difference between voltages AF and FC will be substantially equal to the difference between voltages AG and G0, which difference is directly proportional to the component the voltage of reof current in the load circuit in phase with the voltage of the load circuit.

The voltage of rectifier 62 will therefore exceed the voltage of rectifier 63 by an amount proportional to the in-phase component of the current supplied by machine 5 to the load circuit, and

this voltage difference may be utilized as ameasure of a characteristic of the current in the load circuit. Transformer 49, autotransformer 54 and resistor 56 therefore constitute measuring.

means for producing a voltage which is a measure of a characteristic of the load current and rectificrs 62 and 63 constitute rectifier means energized by a voltage proportional to the in-phase sistor 56 to preserve the desired voltage relations in the circuits if the connections of one of wind ings 53b and 530 are reversed.

Suitable means are provided to produce a voltage which is a measure of the excitation of field winding 9 of machine 5. Such means may consist of a voltage divider, or a saturable reactor 5 having a pair of magnetic cores B5 and 6G sever ally associated with reactance windings ill and E8 and two control windings 69 and T2. The volt-. age divider or the control windings 6i and Y12 may be connected in series with exciter iii and field winding, '3 to be thereby traversed by the current in field winding 9. While the voltage divider or windings $9, 12 may also be connected in parallel with field winding 9, the series connection is preferable as the measure of excitation obtained thereby is independent of variations in the resistance of field winding 9 resulting from temperature variations. The control windings may comprise a variable number of turns depending upon the maximum current intensity in the field winding 9, and where the currents in field winding 9 are large, the control windings may consist of a straight section of bus bar penetrating through the window of the associated reactor oore. Reactance windings 61 and 68 are energized from any suitable alternating current source such as winding 53a of transformer 49 and are connected to any suitable full Wave rectifier l4.

Variations in the current through control windings 69 and i2 operate in a well known manner to produce variations in the reactance ings 61 and 68 and thereby vary the voltage supplied to rectifier l4. Reactor 64 therefore constitutes variable impedance means connected to the field winding means of machine 5 for producing a voltage which is a measure of the excitation of machine 5. A resistor "I3 is connected across the direct current terminals of rectifier 14 to be ener-' gized by a voltage which is ameasure of the excitation of field winding 9 and machine 5.

One terminal of resistor '53 is connected to a direct current terminal of rectifier .63 and the other terminal of resistor 13 is'connected to a direct current terminal of rectifier 62. Owing to the connection of rectifiers 62, 63 and 14, the voltages of rectifiers 63 and 14 are of cumulative polarity and oppose the voltage of rectifier 62.

A saturable reactor 15 comprising two cores 18 and 11, two control winding 18 and 19, and two of windain.

reaetance windings =81 and.8 2, is provided for amplifying the difference between the voltage of rectifier 6.2:and:the voltageof rectifiers Y53 and 74. Reactance'. windings-8i and 82 of reactor IS-are energized. for any suitable alternating current source,, preferably the same source whichenerthezreactor. 15113118" desired 1 characteristic. A- suitv able'iull; wave" rectifier 85 is connected across resistor-.83,.and a resistor. 8"l is connected to the direct currentterminals of rectifier 86; and in. the circuitf field winding 5 to. be therebyienergizedby: a: rectified voltage: proportional. to the voltageadrop in resistor 83;

Control. windings -73 and I9 of reactor. 1.5- are. connectedto. the positive terminals; of rectifiers:

62'and14and2arethereby energized by the difference between the VOItELgGTOf-IECtifiBI'S 63 and Hand thevoltage of rectifier 62; Variations in the currenttraversing control windings l3 and 119' operate in a Well known manner to vary the reactance of windings 8'! and.82 of reactor 15 and thereby vary the voltages ofresistors 83 and 81 to supply a varying voltage to fieldwinding" l5: Current-mayfiow in one direction'through winding 18; winding: 79. resistor 13,1resistor H and rectifierBZ; such direction being determined-by the polarity-of rectifier 62; However, rectifierS-Z prevents the 'fiow of current in such circuit inithc opposite direction.- Reactor-r751 therefore constitutes variableimpedance means difierentially responsive totwovoltagesonly when the first voltage is'greater than the second voltage for. energizing acontrolling means.

The relation between the in-phase component of loadcurrent' as a-function of the minimum excitation ofmachine 5 for stable-operation'of-machine 5 is shown by the graph of Fig.4. The graph shows that the minimum excitation re+ quired 'for' stable operation" of" machine 5 varies directly with the value of the in-phase componentorcurrent supplied by machine 5.

In operation; with machine 5 connected to load conductors 6, land 8 as shown and withthe excitation of field winding 9 above the predetermined minimum value, the circuit elements are so adjusted thatwhen' the-voltage of machine 5 has the desirelcl'value, the cores'of transformers .26" become saturated during parts of'the voltage fcycleand the..voltage of field winding l 5 is represented by agivenpoint 45 on curve '43 of Fig. 2.

If. .for any reason the voltageofmachine 5 decreases. below, the. normal value, the. periodof saturation. oi the cores of transformers 26 be.- comesishorter and the voltages of'capacitorsil.

decreaselmuch more than. the. voltages of Windings 25.;and. 29.. Such .action increasesthe diiferencebetween the voltages of. resistors 3.4..and.3.7'

and. thereby increases the diiferentialvoltage. applied. to field. winding I5 inv a direction. to."- increase the. output voltage-ofexciter H31. Exciter l4 thereby-causes. an increase-in.- the excitationof field winding 90f: ma-chinee5ito preventthe voltage of .machine5 from dropping' materially-below the normal value.

Whenuthe: voltage of: machine. 5 increases abovev the. normal. value the periods of saturation of the coresofi transformers. 26, become longer. and

the. voltages .of capacitors 27. increasevmuch. more However, the; resultantof. such voltages. willnot cause; current. to.-fl0w through control windings. I8 and Til-of reactor becausev rectifier Elblocks. such. a'fiow. With. no current flow in control. windings l8" andiii-,the reactance of windings. 8i andi8'2 remains high and very little voltage .existsaa'cross resistor- 83; The rectified voltageimpressedzonresistor 81 in the circuit ofifieldwinde ing l.5iis.-thereforee.very small and has a neg 1i.-v gible effectzupon. winding l 5.

However, if. a. large load is. suddenly removed; from machine; 5;..thevo1tage of machine 5' in.- creases? and theregulator supplies to field wind.- ing I5: a control voltage which tends to reducev theexcitation. of machine 5. If this action of thexregulatorttends to decrease the excitation. of machine: 5-below theNalue; required for stability, the voltage proportional vtothe in-phase. current. component will exceed the. voltageproportional to the-excitationof machine. 5 and the voltage. o.f= rectifier 62 will exceed the voltages impressed on resistors l|.',.'l3from. rectifiers 63 and Id-,thereby sending a: current. through control windings. 18 and 19. of. a magnitude depending upon the amount .ofitheivoltage excess. This flowof cur.- rent: operates. toslower the reactanceof windings. 8 I v and 82. on reactor. 15 and thereby increases-the. voltages-of resistors 85 and 87. Owing; tothe. connections oflrectifier 86, the voltage impressedonresistor'ii'l: fromirectifier 86 is-of a polarity to. oppose a .control. voltage. from the regulator. which. tends toi decrease: the excitation of machine. 5. Since the maximumamplitude of a control.v0lt-' age of a polarity to decrease the excitation of ma-- .chine- 5: is: limited? by" rectifier 44, as shownfby curve-43 ofaFig..2; thevoltageof resistor 87 rapidlyovercomes this; control. voltage. of predetermined polarity andiincreases: the. excitation of machine. 5' tea value at least equal to that required for.

stable operation;

. Voltage of circuit 6,1,8. If the regulator of maechine' 5 isset for a lower valuethan the regulators;- of the other machines, it will impress on field. winding l5 a. control voltage tending to reduce the excitation ofmachine 5 and such reductionin excitation could lead to. instability in machine. 5.

An additional condition under which thestability of machine? 5 may be threatened is when. machinet-5 issupplying a line on which large amounts of unswitched capacitance are used to. maintainithe: power factor of, the line atareasonablevalue'zin' connection with'a highly induce tiveload. When. this inductive. load is removed from. the. line the unswitched capacitance causes. thegenerator to supply current at a unity or even leading power. factor andth'ereby causes the ma-- chine voltage regulator to reducethe excitation of the machine. to a possibly' dangerous. value. However, .in bothof the above instances the minimumexcitation control device of thisinvention operates. in a manner similar to: that described.

above in connection with the sudden removal of load from machine to supply to field winding I5 a voltage to maintain the excitation of machine 5 at least equal to the minimum value required for stability.

If reactors 65 and i5 were energized from different, power sources, in case of failure of the power for reactor 65, the minimum excitation control device would indicate that the in-phase current component exceeded the measure of excitation of machine 5 and would therefore supply a voltage to field winding iii to increase the excitation of machine 5, with possible resultant damage to the system. By energizing both reactors from the same source as described previously, the possibility of such an occurrence is eliminated.

Although but one embodiment of the present invention has been shown and described, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention or from the scope of the appended claims.

It is claimed and desired to secure by Letters Patent:

1. In a regulating system for a dynamoelectric machine connected to a load circuit, the combination of field winding means for said machine, means for supplying excitation current to said field winding means, control means for controlling the excitation of said field winding means, means including transformer means connected to said load circuit for producing a first voltage which is independent of the voltage of said load circuit and is proportional to the in-phase component of the current supplied by said machine to said load circuit, rectifier means energized by said first voltage, first variable impedance means connected to said field winding means for producing an alternating voltage proportional to the excitation of said field winding means, a rectifier energized by said alternating voltage for producing a second voltage, and second variable impedance means' differentially responsive to the Voltages of said rectifier and said rectifier means only when the voltage of said rectifier means exceeds the voltage of said rectifier for impressing on said control means a voltage of such polarity as to increase the excitation of said field winding means to maintain said excitation above a predetermined minimum value. p

2. In a regulating system for e, dynamoelectric machine connected to a load circuit, the combination of means for supplying excitation current to said machine, means responsive to varia-- tions in the output voltage of said machine from a predetermined value for producing a control voltage having a polarity and magnitude dependent upon the direction and magnitude of said variations, control means responsive to said control voltage for controlling the excitation of said machine, rectifier means connected to said con trol means for limiting the amplitude of said con-- 'trol voltage of a predetermined polarity to decrease the excitation of said machine, means for producing a first voltage which is a measure of a characteristic of the current supplied by said M machine to said load circuit, means for producing a second voltage which is a measure of the excitation of said machine, and means differentially responsive to said first and said second voltages only when said first voltage exceeds said second voltage for impressing on said control means a voltage opposing said control voltage of said predetermined polarity to maintain the excitation of said machine above a predetermined minimum value.

three phasdtransformer having primary phase windings-and secondary phase windings, said primary windings beingener'giz'ed by a measure of the output voltage of said machine, an autotransformer having terminals and a center tap, means connecting said autotransiormer across two of said secondary windings to impress on said autotransformer a voltage in phase with the phase voltage of one of said conductors, impedance means, means for impressing on said impedance means a voltage which is a measure of the current in saidone of said conductors, said impedance means being connected to said center tap, means connected to said impedance means and to said terminals for producing a first voltage proportional to the in-phas'e component of the current supplledby said machine to said one of said conductors, means connected to said field winding means for producing a second voltage proportional to the excitation of said machine, and control means differentially responsive to said first and second voltages only when said first voltage is greater than said second voltage for increasing the excitation of said machine to maintain said excitation above a predetermined minimum value depending on the value of said in-phase component.

4. In a regulating system for a dynamoelectric machine connected to the conductors of a three phase load circuit, the combination of field winding means for said machine, means for supplying excitation current to said field winding means, a three phase transformer having primary phase windings and secondary phase windings, said primary windings being energized by a measure of the output voltage of said machine, an autotransformer having terminals and a center tap, means connecting said autotransformer across two of said secondary windings to impress on said autotransformer' a voltage in phase with the phase voltage of one of said conductors, impedance means, means for impressing on said impedance means a voltage which is a measure of the current in said. one of said conductors, said impedance means being connected to said center tap, means connected to said impedance means and to said terminals for producing a first voltage proportional to a characteristic of the current supplied by said machine to said one of said conductors, means connected to said field winding means for producing a second voltage proportional to the excitation of said machine, and control means differentially responsive to said first and second voltages only when said first voltage is greater than said second voltage for increasing the excitation of said machine to maintain said excitation above a predetermined minimum value.

5. In a regulating system for a dynamoelectric machine connected to the conductors of a three phase load circuit, the combination of field winding means for said machine, means for supplying excitation current to-said field winding means, a three phase transformer having primary phase windings and secondary phase windings, said primary phasewindings being connected to be energized by a measure of output voltage or said machine, an 'autotransformer having terminals xatejo'agitc 1-1 and "a "centertap, "means connecting said fautotransformer across twoiof said secondary-mind- :ings "to :impress onsaidrautotransformer 'a volt- "age which :is in -phase withthe phase' voltage of a one-of said:conductors,lrimpedancemeans, means for impressing "on saidfimpedance -means, a voltage which ,is "a measure of the current said TOIIE of said conductors, *sai'd impedance means being connected ;to said "center" tap, ffirsum'eans connected to ;said impedance :means 'andto 'one of 'said terminals :to 1; produce across said first means a voltage proportional to "the 'sum or the "voltages "of "said *autotransformer and :said impedance means, -secondzineans' connected to said impedancegmeans andnnotherpf said terminals to produce across said second "means a, voltage proportional to the-"difference between the "voltrages :oiisaid impedance means and 'of *said 'autotransformer, means connected "to said first and "said "second means-Eton producing a first voltage which is 'a'measure of thefin-phase :component of .the "current suppliedby "said machine to said 'loadrcircuit'means connectedtosaidfield wind ing-means forproducing a second voltagewhich is -:a measure of the excitation of said machine, 1

and control means fdifferentially "responsive "to said 'lfirstand said :secondvoltage only wnensaid j'first voltage exceedssaid second :voltage .IfOIV increasing the excitation of ,saidmachineLto maintainzsaid excitation above a;predetermined mini :mum value depending on the "value of; said in- "phase component.

"6; In a regulating systemffora ;dynamoelectric machine connected to .a load ,circuit, the com- "bination of field winding means for'said' machine, meansfifor supplying excitation current to said "field winding means, imeansrespons'ive to'varia- "tion in theoutput: voltage of *sai'dmachine :from "a predetermined :value "for producing *a. control -voltage"havingapolarity and magnitude "dependout upon the direction and"magnitude or said variations, controlrmeans responsive -to said :con' *trolvoltage for controlling the supply "of ,excita- *tion currentto saidffieldwindingrmeans, measuring "means connected to said load circuitdor "producing a: first'voltage whichis *a-measureof a -:characteristic of the current supplied by 331d machine to said load circuit, first and second saturable 'reactors -each shaving =a saturating winding and a-reactance winding, "a common said first and second -reactors," means connecting the saturating winding of said first reactor to said field winding means for producing a second voltage which is -a=measureofthe excitation oi said machine, :means connected between said'" measuring means :and the *reactance winding of said tfirst reactor :for :energizing the :saturalting winding .of :said :second :reactor with the difference between said first and said second voltages, zorily when ,said first voltage :exceeds.:said :ssecond circuit, means connectedto said'load circuit for source of current for *therreactance' windings "of 12 pro'ducing .a 'second voltage dependent upon the wourrent and the voltage of said load circuit, means for combining said first and secondveltages "to compensate -'for said load -circuit"voltage to produce a third voltage whichis sulcstantially independent or said "load circuit voltage and which is ameasure-o'f the in-phase component'of the'currentsup-plied by'said machine to saidload circuit, means-connected tosaid field winding meansior-producing a fourth'voltage-which is a measure of the excitation of said machine, and control means differentially responsive "to said third and fourth voltages only when said third voltage exceedssaid fourth voltageforincreasing the excitation of said machine to maintainsaid excitation above a predetermined minimum value.

8. In:a regulating-systemfor a dynamoelectric rnac'hine connected to 'a "load circuit, the- 'combinatioriof fieId -Windingmeansior said machine, mean-s ifor :suipplying excitation current to said field winding -means, means energized :by' :a

measure of the-output voltage of said machine for producing a control voltage having a polarity and a magnitude dependent upon the signan'd :ma'gnitude of variations in said output a voltage ,irom .ia "predetermined value, control :means responsive :totisaid control voltage for controlling the :supply iO'f excitation current ?to said i'field winding :means, means .-.connected to said load circuit for producing :a first avoltage .:dependent ;.upon-the;c.urrent and:the voltage of said. loadicirgcuit, :means :connected ,to :said load tciruuitior producing :a second voltage -dep.endent:;upon the current and "the voltage :of said load circuit, means :for combining said first :and @second voltages to -:compensate ,for said-load circuitvoltage to producei a third voltage which :is substantially independent of said load circuit voltage .and which ,is .-a measure of vthe inphase component .ofathe current supplied by said machinezto said .load circuit, means connected to .saidfield windingi-meansfor producing-a fourth voltage; proportional to the excitation of ,said field Winding imeans, -1'neans differentially responsive to said vthirdand-iourth volt-ages only wvvhenvsaid third -V;O1l3a,g6'-BXC6EC1S .said iourth volta e for impress- .ing on said control means .a voltage of :such polarity-as .to increase the-excitation of said field Windingmeans tomaintain saide-xcitationabove a predetermined minimum valuecdepending .upon the value .of said in-phase component, and a stabilizing transformer having ,a primary winding and a secondarywindin-g,.said primary. .wind ing being. connected'ito saidfield winding means 7 ,and said secondary winding being connected to .saidtcontrol means.

9. In aregulating system for ajdynamoelectric machine connected to the conductors of "a load circuit, the combination ofjfie'ld winding ,means for said machine, meansffor supplying excitation current to said .Tfield winding means, means energized by a measure of the output voltage of said machine for producing 'a control'voltage havinga polarity and ;a magnitude dependent upon thesign and magnitude of variations in said output voltage frame predetermined value, control means responsive to said :control voltage for controlling the supply of excitation current to :said field-winding -means, means connected to said load circuit-for producing a first voltage proportional tothe vector sum of a voltagepro- 'portional to 'thecurrent in one of said-conductors of said loadc'ircuitand the =voltage of said conductor to neutral, means connected to said load circuit-for producing a second-voltage-propor- 13 tional to the vector difference between a voltage proportional to said current in said one conductor and said voltage of said conductor to neutral, means including rectifier means for producing a third voltage proportional to the difference between the absolute values of said first and said second voltages, whereby said third voltage is substantially independent of the magnitude of said load circuit voltage and is a measure of the in-phase component of said load circuit current, means connected to said field winding means for producing a fourth voltage proportional to the excitation of said field winding means, and means differentially responsive to said third and fourth voltages only when said third voltage exceeds said fourth voltage for impressing on said control means a voltage of such polarity as to increase the excitation of said field winding means to maintain said excitation above a predetermined minimum value depending upon the value of said in-phase component.

10. In a regulating system for a dynamoelectric machine connected to the conductors of a load circuit, the combination of field winding means for said machine, means for supplying excitation current to said field winding means, means energized by a measure of the output voltage of said machine for producing a control voltage having a polarity and a magnitude dependent upon the sign and magnitude of variations in said output voltage from a predetermined value, control means responsive to said control voltage for controlling the supply of excitation current to said field winding means, means including rectifier means connected to said control means for limiting the amplitude of said control voltage when said control voltage has the predetermined polarity which decreases the excitation of said field winding means, means connected 14 to said load circuit for producing a first voltage proportional to the vector sum of a voltage proportional to the current in one of said conductors of said load circuit and the voltage of said conductor to neutral, means connected to said load circuit for producing a second voltage proportional to the vector difference between a voltage proportional to said current in said one conductor and said voltage of said conductor to neutral, means for producing a third voltage proportional to the difference between the absolute values of said first and second voltages, whereby said third voltage is substantially independent of the magnitude of said load circuit voltage and is a measure of the in-phase component of the current in said load circuit, means connected to said field winding means for producing a fourth voltage proportional to the excitation of said field winding means, means difi'erentially responsive to said third and fourth voltages only when said third voltage exceeds said fourth voltage for impressing on said control means a voltage opposing said control voltage of said predetermined polarity to maintain the excitation of said machine above a predetermined minimum value, and a stabilizing transformer having a primary winding and a secondary winding, said primary winding being connected to said field winding means and said secondary winding being connected to said control means.

DANIEL J. SIKORRA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,459,640 Griscom et a1. Jan. 18, 1949 2,478,623 Crary et a1. Aug. 9, 1949 

